The present invention relates to a system and a method for monitoring an engine emission control system, and in particular to monitoring a NOx sensor coupled downstream of an emission control device.
Internal combustion engines are typically coupled to an emission control device known as a three-way catalytic converter (TWC) designed to reduce combustion by-products such as carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx). Engines can operate at air-fuel mixture ratios lean of stoichiometry, thus improving fuel economy. For lean engine operation, an additional three-way catalyst commonly referred to as a Lean NOx Trap (LNT), is usually coupled downstream of an upstream catalytic converter. The LNT stores exhaust components, such as oxygen and NOx, during lean operation. Continued lean operation eventually saturates the LNT with the selected exhaust gas constituents. After the LNT is filled to a predetermined capacity, stored exhaust gas constituents are typically reduced and released (purged) by switching to rich or stoichiometric operation, i.e., by increasing the ratio of fuel to air and thereby increasing the amount of reductant such as hydrocarbon (HC) present in the exhaust gas mixture entering the LNT. Once the purge is completed, lean operation resumes again.
One way of determining when to purge the LNT is by installing a sensor capable of measuring an amount of NOx in the exhaust gas exiting the LNT. Typically, the sensor is monitored to determine when the amount of tailpipe NOx emissions in grams/mile exceeds a predetermined threshold in order to discontinue lean operation. Over time, the performance of the NOx sensor can deteriorate due to such causes as contamination or electrical degradation. This can result in an incorrect determination of when to end lean operation, and may result in lean operation being too long or too short, thus degrading emission control or fuel economy. It is therefore desirable to monitor the performance of the emission control system, and in particular to detect the degradation of the NOx sensor.
One method of NOx sensor monitoring is described in U.S. Pat. No. 5,426,934, wherein a NOx catalyst is coupled to an upstream (post-catalyst) and a downstream (pre-catalyst) NOx sensor. Only the upstream (pre-catalyst) NOx sensor is monitored for deterioration. The method includes comparing the ratio of the NOx sensor output during lean operation (NOxlean) and the NOx sensor output at stoichiometry (NOxrich) to a predetermined value. A decrease in the ratio below the predetermined value is indicative of sensor deterioration.
The inventors herein have recognized a disadvantage with this approach. Namely, this method would not work for a post-catalyst NOx sensor since the LNT stores NOx during lean operation, and therefore the NOxlean signal downstream of the LNT will be attenuated. Therefore, the ratio of the prior art cannot be used as an indicator of post-catalyst sensor deterioration.
It is an object of the present invention to provide a system and a method for determining degradation in an emission control system.
In carrying out the above object and other objects, features and advantages of the present invention, a system and a method for determining degradation in an emission control system comprising an exhaust gas aftertreatment device having a downstream sensor coupled to it, include: changing an air-fuel mixture of an exhaust gas entering the device; and determining degradation of the sensor based on a response of the sensor to said change in said air-fuel ratio.
For example, in accordance with one embodiment of the present invention, the inventors have recognized that once lean operation is discontinued, and the air-fuel ratio of the exhaust gas entering the LNT is switched to rich, there is a significant temporary increase in the amount of NOx in the exhaust gas exiting the LNT, which is typically reflected by a surge in the NOx sensor output if the NOx sensor is not degraded. Further, the inventors have recognized that a degraded sensor will not detect this surge in the amount of NOx exiting the LNT. The NOx surge can occur within a predetermined time period following the lean to rich transition, and may sometimes happen after the purge is completed, and the air-fuel ratio is changed back to lean. It is partially due to a significant temperature increase of the LNT resulting from the increased amount of reductant entering it during the purge. In other words, increased LNT temperature contributes to increased NOx in the exhaust gas exiting the LNT. Therefore, under the present invention, the performance of the NOx sensor can be monitored by monitoring its response to the NOx surge for a predetermined time period following the switch from lean to rich operation.
In accordance with another feature of the present invention, in an exemplar embodiment, a system and a method for determining degradation in an emission control system comprising an exhaust gas aftertreatment device having a downstream sensor coupled to it, include: operating the engine at an air-fuel ratio lean of stoichiometry to store an exhaust gas constituent in the device; temporarily switching to an air-fuel ratio rich of stoichiometry to release said stored exhaust gas constituent from the device; reading an output of the sensor for a predetermined period following said temporary switch to determine a maximum value and a minimum value of said reading; and comparing a ratio of said maximum value and said minimum value to a predetermined threshold.
Therefore, according to this embodiment, it is possible to detect deterioration in the sensor by determining a maximum and a minimum value of the sensor reading following a switch to rich mode of operation, and by comparing the ratio of the two to a predetermined threshold indicative of a borderline sensor performance. In other words, a sensor that is not deteriorated will detect a surge in the amount of NOx exiting the LNT in response to a switch to rich operation, and the ratio of the maximum value to a minimum value taken during a predetermined period following the switch will be above a threshold amount. On the other hand, a deteriorated sensor will not detect the surge, and the ratio of maximum to minimum value will be below a predetermined threshold.
An advantage of the above aspects of invention is optimized lean running time, increased fuel economy, and improved emission control.
The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.